1. Field of the Invention
The invention relates to an amplifying circuit arrangement including a signal source having a high-ohmic output impedance and a low-ohmic amplifier.
2. Background Information
Such circuit arrangements serve, for example, in optical receivers to convert optical signals into electrical signals and to preamplify these electrical signals.
In optical transmission systems having an optical receiver which is highly sensitive over a relatively narrow frequency range and includes a light sensitive component (photodiode) and an amplifier connected to its output, the performance level of an optical receiving signal to be processed by an optical receiver can vary greatly. The receiver must therefore be able to detect without errors very low as well as very high light levels. The lower limit is called receiver sensitivity. It is essentially a function of the noise characteristics of the detector element which is configured as a light sensitive component. The receiver sensitivity further depends on the circuitry of the amplifier employed within the optical receiver.
The publication by J. A. Geisler, entitled "Optical Fibres" (EPO Applied Technology Series; Volume 5), 1986, Part III, Chapter III, pages 437-526, discloses a multitude of optical receivers. The optical receivers described there exhibit complicated circuitry for the amplifier connected to the output of the light sensitive component, with the amplifier being realized as a high impedance amplifier or as a transimpedance amplifier. If one desires a large dynamic range, optical receivers including a transimpedance amplifier are employed with preference. Such an optical receiver including a transimpedance amplifier is disclosed, for example in DE-A1 3,938,097. Transimpedance amplifiers have the highest possible impedance at their input and can be used over a broad frequency range. They perform the function of a current/voltage converter which converts the electrical current from the light sensitive component into a voltage. The drawback of such a type of amplifier is a resulting deterioration of the signal to noise ratio which brings with it a limitation of the receiver sensitivity.
Prior art broadband amplifying circuit arrangements are used, for example, for optical broadband receivers in optical CATV (cable television) and digital communication systems. A cost-effective optical broadband receiver for analog amplitude modulated signals having a band range from 50 MHz to 550 MHz, is disclosed in European Patent EP 0,372,742 A2. The prior art receiver comprises a light sensitive component, which, for impedance matching, is connected to a broadband, low-ohmic amplifier by way of a transmitter.
The drawback of the prior art receiver is that the light sensitive component, despite impedance matching, demonstrates a poor output reflection factor which causes severe harmonic distortion in the higher order harmonics in the amplifier.